This invention relates to crystalline galliosilicates and is particularly concerned with a crystalline galliosilicate molecular sieve having the mordenite structure and methods of producing such a molecular sieve.
Zeolites are well known natural and synthetic molecular sieves that can be defined as crystalline, three-dimensional aluminosilicates consisting essentially of alumina and silica tetrahedra which interlock to form discrete polyhedra. The polyhedra are interconnected to form a framework which encloses cavities or voids that are interconnected by channels or pores. The size of the cavities and pores will vary depending on the framework structure of the particular zeolite. Normally, the cavities are large enough to accommodate water molecules and large cations which have considerable freedom of movement, thereby permitting sorption, reversible dehydration and ion exchange. The dimensions of the cavities and pores in a zeolite are limited to a small number of values and can vary from structure to structure. Thus, a particular zeolite is capable of sorbing molecules of certain dimension while rejecting those of dimensions larger than the pore size associated with the zeolite structure. Because of this property zeolites are commonly used as molecular sieves.
In addition to their molecular sieving properties, zeolites show a pronounced selectivity toward polar molecules and molecules with high quadrupole moments. This is due to the ionic nature of the crystals which gives rise to a high nonuniform electric field within the micropores of the zeolite. Molecules which can interact energetically with this field, such as polar or quadrupolar molecules, are therefore sorbed more strongly than nonpolar molecules. This selectivity toward polar molecules is the unique property of zeolites which allows them to be used as drying agents and selective sorbents.
The pore size of a zeolite can vary from about 2.6 Angstroms for analcime to about 10.0 Angstroms for zeolite omega. The term "pore size" as used herein refers to the diameter of the largest molecule that can be sorbed by the particular zeolite or other molecular sieve in question. The measurement of such diameters and pore sizes is discussed more fully in chapter 8 of the book entitled "Zeolite Molecular Sieves," written by D. W. Breck and published by John Wiley & Sons in 1974, the disclosure of which book is hereby incorporated by reference in its entirety. The pore size range of 2.6 to 10.0 Angstroms is particularly suited for molecular separation and catalytic processing. Analcime will sorb ammonia while excluding larger molecules whereas zeolite omega will sorb perfluorotributyl amine [(C.sub.4 F.sub.9).sub.3 N] while excluding any molecule having a diameter greater than about 10.0 Angstroms. All of the other approximately 150 zeolites now known have pore sizes falling within the range between 2.6 and 10.0 Angstroms.
In addition to their use as drying agents and selective sorbents, zeolites are widely used as components of chemical conversion catalysts. As found in nature or as synthesized, zeolites are typically inactive because they lack acid sites. In general, acid sites are created by subjecting the zeolite to an ion exchange with ammonium ions followed by some type of thermal treatment which creates acid sites by decomposing the ammonium ions into gaseous ammonia and protons. Activated zeolites have been used in many types of chemical conversion processes with the smaller pore zeolites being used to selectively sorb and crack normal and moderately branched chain paraffins.
Because of the unique properties of zeolitic molecular sieves, there have been many attempts at creating new molecular sieves by either substituting an element for the aluminum or silicon present in zeolitic molecular sieves or adding another element in addition to the aluminum and silicon. The term "zeolite" or "zeolitic" as used herein refers to molecular sieves whose frameworks are formed of substantially only silicon and aluminum atoms in tetrahedral coordination with oxygen atoms. One such class of new molecular sieves that has been created is that in which all the framework aluminum has been replaced by gallium. Specifically, it has been reported in European Patent Application No. 84304025.4 published on Jan. 2, 1985 as Publication No. 0 130 013 that a galliosilicate molecular sieve having the mordenite structure has been synthesized. The synthesis process as illustrated in Examples 1 and 2 involves the use of diethanolamine as a templating or directing agent. According to the examples, the hydrogel from which the galliosilicate mordenite was synthesized contained 32.4 moles of diethanolamine and 850 moles of water. The use of such large amounts of templating agent, which tends to be relatively expensive, greatly adds to the cost of the synthesized sieve while the use of such relatively large quantities of water results in less yield of sieve when the hydrogel is subjected to crystallization. Thus, the process disclosed in the European application has at least two major drawbacks that result in a relatively expensive galliosilicate mordenite.
Accordingly, it is one of the objects of the present invention to provide a less expensive crystalline, galliosilicate molecular sieve with the mordenite structure, and a method for preparing such a molecular sieve, which can be useful in many types of chemical conversion processes, particularly hydrocarbon conversion processes and processes to control atmospheric emissions of nitrogen oxides. This and other objects of the invention will become more apparent in view of the following description of the invention.